Our U.S. Pat. No. 4,298,390 discloses the production of spontaneous opal glasses wherein the opacity is the result of crystallinity in the glass and Ba.sub.2 F(PO.sub.4) constitutes the predominant crystal phase. Those glasses are asserted to manifest softening points in excess of 710.degree. C., a white opacity, excellent chemical durability, and to consist essentially, expressed in terms of weight percent on the oxide basis, of 6-10% Na.sub.2 O, 1-6% K.sub.2 O, 4-11% BaO, 9-18% Al.sub.2 O.sub.3, 1-5% B.sub.2 O.sub.3, 50-70% SiO.sub.2, 3.5-7% P.sub.2 O.sub.5, 1-4% F, and optionally up to 3.5% CaO and/or up to 5% total of MgO and/or SrO.
The patent explains that those glasses are characterized by a two-stage liquidus phenomenon; viz., a high temperature cloudiness or opacification, termed an emulsion liquidus or liquid-liquid phase separation, and the normal crystalline opal liquidus. Analysis of the phase separation found it to be rich in Na.sub.2 O, BaO, P.sub.2 O.sub.5, and F. X-ray diffraction analysis of the crystalline opal phase identified the predominant crystal phase to be of a Ba.sub.2 (OH)PO.sub.4 type. Nonetheless, because X-ray analysis does not distinguish between fluoride and OH, it was assumed that fluoride substituted for OH in the crystal, thereby leading to the crystals being described as Ba.sub.2 F(PO.sub.4). Minor amounts of NaBaPO.sub.4 and other unidentifiable species were also detected.
Culinary ware and tableware are thermally tempered to improve the mechanical strength and thermal shock resistance thereof. Unfortunately, breakage of such ware produced according to the disclosure of U.S. Pat. No. 4,298,390 was not infrequently experienced during the thermal tempering procedure. Microscopic examination of the crystal phase indicated that a substantial number of the crystals attained relatively large dimensions with inclusions of such crystals exhibiting diameters in excess of 0.001" (.about.25 microns). Ware containing such large inclusions do not survive the thermal shock inherent in the air chill tempering process, viz., about 800.degree. C. to room temperature.
A further and very extensive investigation of those glasses has determined that the crystallization mechanism is more complex than originally conjectured. This study demonstrated that, instead of a single crystallization liquidus, as discussed in U.S. Pat. No. 4,298,390, there appears to be a first or high temperature crystallization liquidus and a second or low temperature crystallization liquidus. Hence, at the high temperature crystallization liquidus at least one species of apatite-type crystal [classical formula Ca.sub.10 F.sub.2 (PO.sub.4).sub.6 ] precipitates out of the molten glass, and at the low temperature crystallization liquidus at least one other species of apatite-type crystal is generated, as confirmed by X-ray diffraction data. Much solid solution is possible in the apatite structure which manifests itself in very minor changes in the X-ray diffraction pattern; i.e., the overall pattern is relatively indistinguishable from the general appearance of classical apatite. Hence, as used herein, apatite includes such solid solution crystals.
It was noted in U.S. Pat. No. 4,298,390 that the emulsion liquidus and crystallization opal liquidus data were obtained utilizing a hot stage microscope composite apparatus. Such apparatus relies upon the sensitivity of the eye of the observer and, therefore, involves a significant measure of subjectivity in the reported determinations. Accordingly, to remove the element of subjectivity, the emulsion and low temperature crystallization liquidi were determined on a number of the working examples of the patent employing laser reflectance measurements, that technique being founded in conventional laser reflectance spectroscopy. Thus, those liquidi are readily gained from laser reflectance or back scattering/temperature curves. The high temperature crystallization liquidus cannot normally be directly obtained from such curves because that deflection is hidden in the steep slope brought about by the emulsion phenomenon. Consequently, the high temperature crystallization liquidus is read from the laser reflectance or back scattering/temperature curve in the derivative mode.
Table A records the emulsion, high temperature crystallization, and low temperature crystallization liquidi (in .degree.C.) measured on several of the working examples provided in Table I of the patent. The Example Nos. reflect those in Table I of the patent.
TABLE A __________________________________________________________________________ 1 2 3 7 8 9 10 12 15 17 18 __________________________________________________________________________ Emulsion Liquidus 1380 1400 1390 1380 1380 1370 1395 1380 1390 1400 1385 High Temp. Liquidus 1350 1365 1370 1350 1345 1345 1360 1350 1375 1370 1360 Low Temp. Liquidus 875 860 610 700 760 725 710 820 900 740 770 __________________________________________________________________________
As can be observed, the interval between the emulsion liquidus and the high temperature crystallization liquidus is less than 50.degree. C., often no more than 25.degree. C. The above-mentioned extensive investigation of those glasses has indicated that, to prevent the formation of those large crystalline inclusions which can cause mechanical breakage of ware, the temperature range between the emulsion liquidus and high temperature crystallization liquidus must be expanded. That is the principal objective of the present invention.